dietary salt in postural tachycardia syndrome

Conditions: Postural Orthostatic Tachycardia Syndrome

Summary: patients with pots may not adequately expand their plasma volume in response to a high-sodium (na+) diet. mechanisms involved in the regulation of plasma volume, such as the renin-angiotensin-aldosterone system and renal dopamine, may be impaired in pots and may respond inappropriately to changes in dietary sodium.the purpose of this study is to determine (1) whether a high dietary sodium level appropriately expands plasma volume in pots; (2) whether plasma renin activity and aldosterone are modified appropriately by changes in dietary sodium in pots; and (3) whether patients with pots have improvements in their orthostatic tachycardia and symptoms as a result of a high dietary sodium level.

study day 1 * start 150 milliequivalents (meq) na+/day diet (pots patients as inpatients; healthy control subjects with clinical research center (crc)- provided outpatient diet) * start a 24h urine collection (for na+, k+, cr, fractionated catecholamines) * blood work * blood volume - carbon monoxide rebreathing study day 2 * complete 24h urine * start study diet (10 meq na+/day or 300 meq na+/day in a random order) after 3 meals of 150 meq na+/day are complete; water ad lib study day 3 - 5 * continue study diet; water ad lib * on day 5, a 24 hr holter combined ecg monitor will be placed on the subjects. study day 6 * continue study diet; water ad lib * remove 24h holter combined ecg monitor and bp monitor from subject * start a 24h urine collection (for na+, k+, cr, fractionated catecholamines) * complete questionnaires * nothing by mouth (npo) after midnight for study next day study day 7 (big day) * awaken early (\~6am) to void (still collecting 24h urine) * patient returns to bed, iv catheter inserted * posture study (in morning; between 7-8am ideally) * blood pressure and heart rate will be measured while supine and then while standing for up to 30 minutes * we will draw 3 tablespoons of blood in each body position to measure hormones that regulate blood pressure and blood volume * hormones to verify the subject's phase of menstrual cycle * serum/plasma aliquots for future analysis * subjects will rate symptoms during supine period and at end of stand using vanderbilt orthostatic symptoms score (voss) * total blood volume (daxor)- using injection of iodinated i-131 tagged human serum albumin nominally 25 micro-ci of radiation blood samples drawn through iv catheter before injection and for \~30 minutes post-injection (total - 25 ml) * this will be done after supine assessment, but before standing the subject up * autonomic function test with cardiac output and brief tilt * the subject will be tilted up to 60-75 degrees head-up tilt for up to 10 minutes to measure the changes in heart rate and blood pressure and symptoms with upright challenge. * blood volume - carbon monoxide rebreathing * exercise capacity test (in the afternoon) will estimate maximal oxygen consumption (vo2 max) this test will be conducted on a stationary bicycle. effort will be gradually increase while expired air is measured during exhaustive physical work. all procedures are repeated at least a month later with the 2nd level of dietary salt. (randomized to high or low salt to the first phase, the second phase is the remaining level)

Eligibility criteria:
inclusion criteria: * diagnosed with postural tachycardia syndrome by the vanderbilt autonomic dysfunction center * increase in heart rate ≥30 beats/min with position change from supine to standing (10 minutes) * chronic symptoms consistent with pots that are worse when upright and get better with recumbence * age between 18-50 years old * non-smokers * premenopausal patients with pots and healthy volunteers * only female participants are eligible. * since 80-90% of pots patients are female, and there can be differences in measures with the menstrual cycle, including a small number of males might introduce a significant amount of noise. * able and willing to provide informed consent exclusion criteria: * smokers * overt cause for postural tachycardia, i.e., acute dehydration * significant cardiovascular, pulmonary, hepatic, or hematological disease by history or screening results * pregnant (positive pregnancy test) or breastfeeding * hypertension defined as supine resting bp\>145/95 mmhg off medications or needing antihypertensive medication * other factors which in the investigator's opinion would prevent the participant from completing the protocol, including poor compliance during previous studies or an unpredictable schedule * unable to give informed consent

Keywords: pots orthostatic intolerance orthostatic tachycardia